1. Field of the Invention
The present invention relates to a stabilization method for olefin polymers. More particularly, the present invention relates to a method for stabilizing olefin polymers which comprises treating olefin polymers with a specific treating agent, the olefin polymers being obtained by polymerization of olefins in the presence of a catalyst containing a compound of a transition metal of Groups IVa to VIa of the Periodic Table and a hydride or an organic compound of a metal of Groups II to III of the Periodic Table, i.e., a Ziegler type catalyst.
2. Description of the Prior Art
The physical properties of olefin polymers which are obtained by olymerizing olefins such as ethylene, propylene or the like using a Ziegler type catalyst are deteriorated by light or heat. In order to prevent this deterioration, a stabilizer (e.g., a heat stabilizer, an ultraviolet absorbent, etc.) is added to the polymer prior to a step such as a granulation step in which the polymer is to be heated, as described in, for example, British Pat. No. 980,468.
However, some catalyst which uwas used for the polymerization is retained in the polymers (for brevity, hereinafter merely referred to as "residual catalyst"), and the residual catalyst in combination with a stabilizer causes a deterioration of the quality of the olefin polymers, such as discoloration, thermal deterioration, bad smell or the like, to occur.
Therefore, olefin polymers containing residual catalyst are generally neutralized with an alkali immediately after polymerization and washed with a large amount of an alcohol or water thereby to sufficiently dissolve and remove the residual catalyst in the polymers followed by addition of a stabilizer thereto as described in, for example, U.S. Pat. No. 3,644,321.
However, such a removal operation of the residual catalyst is extremely complicated, and in practice industrially, the cost of this treatment step is expensive, which is remarkably disadvanageous from the economical standpoint. Therefore, the development of a simpler and cheaper treatment method for removal of the residual catalyst is desired.
As a result, various methods for stabilizing olefin polymers containing residual catalyst without removing the residual catalyst from the olefin polymers have been proposed. For example, methods for contacting the olefin polymers with an epoxy group-containing compound, a hydroxy group-containing organic compound, an alkyl ester of a carboxylic acid or the like, oxygen or steam as described in Japanese Patent Publication No. 38035/75, or a method for treating the olefin polymers with an ester of phosphoric acid (as described in Japanese Patent Publications Nos. 38035/74 and 26/66), an ester of silicic acid (as described in Japanese Patent Publication No. 18460/74 or an ester of boric acid (as described in Japanese Patent Publication Nos. 38035/74 and 26/66) are known.
However, the degree of color improvement of the olefin polymers using the above-described material is poor, and the above-described methods can not be satisfactorily used on an industrial scale.
As a result of extensive investigations on obtaining olefin polymers for which an operation for removal of the polymerization catalyst residues is not required, a discoloration, even if the polymers are subjected to a treatment step involving heating such as granulation, does not occur, and the physical properties of the polymers are not deteriorated (e.g., a cleaving of the high molecular chains by oxidation, heating, etc.), it was found that olefin polymers can be stabilized by treating the same with a specific compound and the present invention was accomplished.